Gas assisted flotation (GAF) primarily in the form of dissolved air flotation (DAF) cells have been in use for years as a primary means of separating solids from liquids. In recent years, dissolved air flotation, has found wide acceptance in the treatment and pre-treatment of various waste waters prior to discharge or recycle. The advantages of a flotation process over a sedimentation process for liquid-solids separation are well known and some of them are set forth below. However, one disadvantage common to all dissolved air flotation cells currently in use is the inability to remove the sludge accumulation from the cell without taking significant amounts of the liquid with it.
Dissolved air flotation cells such as described in U.S. Pat. Nos. 3,452,869 and 4,022,695 have competed with gravity sedimentation for the separation of precipitated or other suspended solids from liquids or solutions for a long time.
Dissolved air flotation can have many advantages over gravity sedimentation. One of the greatest is space. Light coagulated-flocculated solids, such as various chemical precipitates, are slow to settle and thicken in their liquid media by gravity alone. It is therefore necessary to build relatively large settling tanks or thickeners to accomplish a good clean separation.
On the other hand a DAF cell or unit that will handle the same flow rate can be much smaller because the rise rate of floc attached to air bubbles is typically four or five times greater than the settling rate of the same floc without air. Therefore, a DAF unit will always be much smaller than a sedimentation unit for the same volume throughput. DAF cells can be operated at high surface loading, thus small and quite shallow units are possible. Another advantage of DAF cells over sedimentation can be fast start-up with steady water quality being achieved quickly. Cost is also favorable in many cases.
One major disadvantage of all existing DAF cell configuations, however, is the inability to separate the floating solids (floc) from their liquid media without removing an excess of liquid. This results in a very dilute sludge, often less than 0.2 percent solids. Such a dilute sludge is not an ideal feed for a filter or de-watering unit. This has been a major drawback to the DAF process for liquids-solids separation since its inception.
Currently there are methods of removing floc from a DAF cell in use, such as an inclined continuous belt where the floc is picked up on the belt and excess water runs back into the cell. The floc can then be scraped off the belt into a receiver outside of the cell. Another available method is to use an inclined screw which is analogous to the a belt system. With the screw system, scoops of various design are in use to make the separation. Still another method involves a centrally-mounted revolving weir in a circular cell where the floc is carried off, but always with a significant amount of liquid. Sometimes rotary paddles or stationary scrapers are used to decrease the amount of solution carried away along with the true floating sludge.
In recent years DAF has been used increasingly in the treatment or pre-treatment of many and varied waste waters, but always there has been a need for the production of a floc with a higher solids content which would create less sludge for disposal and make a more efficient feed for a filter or other de-watering device.